1. Field of the Invention
The present invention relates to a method for preparing a compound oxide superconductor thin film, and more particularly, it relates to a method for preparing a compound oxide superconducting thin film of homogeneous composition having a high critical temperature of superconduction.
2. Description of the Prior Art
Superconduction, which is explained as phase transition of electrons, is a phenomenon by which a conductor loses all resistance to electric current under specific conditions to present complete diamagnetism.
Various types of superconducting devices have been proposed and developed in the field of electronics, to which the superconducting phenomenon is typically applied. A typical example of such superconducting devices is that employing the Josephson effect, through which a quantum effect is macroscopically developed by applied current when superconductive materials are weakly joined with each other. A tunnel junction type Josephson device, which has a small energy gap between superconductive materials, is expected as a switching device of an extremely high speed with low power consumption. Further, application of the Josephson device to a supersensitive sensor for magnetic fields, microwaves, radioactive rays or the like is also expected since the Josephson effect for electromagnetic waves or magnetic fields is developed as a correct quantum phenomenon.
Development of a superconducting device for a very high speed computer is demanded since power consumption per unit area has approached the limit of cooling ability, while application of a superconductive material having no current loss to an interconnection member is demanded with improvement in degree of integration of an electronic circuit.
In spite of various efforts, superconduction critical temperatures T.sub.C of conventional superconductive materials remained below 23 K, that of Nb.sub.3 Ge, for many years. However, it was discovered in 1986 that sintered bodies of [La,Ba].sub.2 CuO.sub.4, [La,Sr].sub.2 CuO.sub.4 and the like superconduct at higher critical temperatures, to open the possibility of non-cryogenic superconduction. It has been observed that these substances superconduct at critical temperatures of 30 to 50 K, which are extremely higher than those of the conventional materials. Some of the substances have superconducted at temperatures exceeding 70 K.
It has been announced that a compound oxide called YBCO, which is expressed as Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x, is a 90 K level superconductor. Further, compound oxides of Bi--Sr--Ca--Cu and Tl--Ba--Ca--Cu, which superconduct at temperatures exceeding 100 K, are so chemically stable that the same are not deteriorated in superconductivity with time, dissimilarly to YBCO or the like.
Superconductivity of a superconductor prepared by any such compound oxide is greatly influenced by oxygen deficiency in crystal. If oxygen deficiency in crystal is improper, the critical temperature is reduced and the difference between an onset temperature and a temperature at which electrical resistance completely disappears increases.
In general, a superconductor thin film of the aforementioned compound oxide has been provided by forming a film by physical deposition such as sputtering with a deposition source of an oxide generated by sintering or the like and thereafter performing heat treatment in an oxygen atmosphere or exposing the film to oxygen plasma.
Such a compound oxide superconductor material is easily deteriorated in superconductivity particularly when the same is provided in the form of a thin film. This is conceivably because oxygen deficiency in crystal of the compound oxide superconductor is improper during in formation of the thin film, to cause a significant problem in use of the compound oxide superconductor material.